Lift off cylinder for axial piston hydraulic pump

ABSTRACT

A positive displacement hydraulic pump/motor assembly ( 1 ) includes a rotary cylinder block ( 3 ) having a central axis ( 5 ) and a generally circular array of cylinders ( 6 ) disposed in parallel relationship around the axis. A corresponding plurality of axial pistons ( 10 ) is reciprocably disposed within the respective cylinders. A drive shaft ( 12 ) effects rotation of the cylinder block about the central axis and a drive plate ( 15 ) is disposed at one end of the cylinder block ( 3 ) to effect sequentially staggered reciprocation of the pistons ( 10 ) in response to rotation of the cylinder block. A stationary valve plate ( 20 ) is disposed at an opposite end of the cylinder block. The valve plate ( 20 ) includes a valve face ( 21 ) adapted for sliding rotational engagement with a complementary mating face ( 22 ) formed on the cylinder block. The valve plate further includes at least one inlet port ( 24 ) adapted for fluid communication with a source of hydraulic fluid and at least one outlet port ( 26 ) adapted for fluid communication with an hydraulic load. The ports ( 24, 26 ) are disposed such that in use, hydraulic fluid is progressively drawn into the cylinders in sequence as the respective pistons are displaced away from the valve plate and subsequently expelled from the cylinders as the pistons are progressively displaced toward the valve plate. The pump/motor assembly further includes lift-off means in the form of an hydraulic lift-off piston ( 40 ) and cylinder ( 41 ), selectively operable to displace the cylinder block marginally away from the valve plate, thereby to minimise rotational resistance between the respective mating valve faces ( 21, 22 ) under predetermined operational conditions.  
     The pump/motor assembly is adapted for incorporation into an energy management system operable in a driving mode, a braking mode and a neutral motor to provide supplementary drive from regenerative braking in a vehicle.

FIELD OF THE INVENTION

[0001] The present invention relates generally to hydraulic motors andpumps, and more particularly to positive displacement axial pistonmotors and pumps.

[0002] The invention has been developed primarily for use with apump/motor assembly which forms part of a regenerative drive system(“RDS”), and will be described predominantly hereinafter in thatcontext. It should be appreciated, however, that the invention is notlimited to this particular field of use, being readily adaptable to anyaxial piston hydraulic motor or pump for use in virtually anyapplication.

BACKGROUND TO THE INVENTION

[0003] In the present context, the invention has been developed morespecifically as an improvement to the RDS described by the presentapplicant in an earlier patent application filed via the PatentCooperation Treaty (PCT) as international application No.PCT/AU99/00740, the full contents of which are hereby incorporated byreference.

[0004] As previously described in that earlier patent application, theRDS is based upon a positive displacement pump/motor arrangementincorporating a cylinder block which houses a cylindrical array ofaxially reciprocating pistons. In one preferred embodiment, the cylinderblock and valve face are coaxially disposed around the primary driveshaft, thereby avoiding the need for intermediate gearing, chains, beltsor other transmission elements. When used in conjunction with a suitableaccumulator, the resultant regenerative drive system provides apractical and commercially viable system for harnessing the previouslywasted braking energy of a vehicle, storing this energy, andsubsequently releasing it back into the drive train as required underconditions of acceleration, load, or gear change transitions.

[0005] This RDS arrangement significantly improves the overallefficiency of the engine and power transmission systems of the vehicle.The RDS system also conveniently acts as an efficient auxiliary brakingmechanism in the energy accumulation mode. The system as described,however, is subject to several significant limitations, many of whichare common to previously known axial piston hydraulic motors and pumps.

[0006] In this regard, one significant limitation in the pump/motorassembly as previously described, relates to the inherent dragassociated with the seals, bearings, valve faces and other elements thatare in direct sliding contact with each other, as the cylinder block andother components in the rotational group rotate with respect to thevalve plate, housing and other stationary components of the system. Inthe particular RDS unit described, these frictional drag forces arereduced by virtue of the fact that most of the components in directcontact with one another “float” on a film of oil, which is oftenpressurised. Nevertheless, a residual drag factor remains, whichconsumes power, generates heat, and compromises the potential efficiencyof the system.

[0007] From this perspective, the present invention is concerned morespecifically with the drag normally attributable to the directcommunication between the rotational group including the cylinder block,and the valve plate. In the pump/motor assembly and RDS unit aspreviously described in the earlier patent application referenced above,a “hold-down” spring is disposed resiliently to bias the cylinder blockaxially into face-to-face sliding engagement with the valve plate.Because of the relatively high pressures generated within the pump/motorassembly, the hold-down spring is required to exert a relatively highdegree of axial force on the cylinder block in order to preventexcessive fluid leakage between the block and the valve plate.

[0008] While effective in preventing excessive leakage, the relativelyhigh axial force between the cylinder block and the valve plate causessignificant frictional drag between these components as they sliderotationally relative to one another, separated only by a thin film ofpressurised oil. The associated inefficiency is particularly significantin situations where the pump/motor assembly is operating in a neutral or“free-wheel” mode, in which the unit is neither pumping nor driving butis nevertheless rotating, often at high speed, as a result of directconnection with a rotary source of power or load.

[0009] While applying to some extent in almost any application of axialpiston hydraulic motors and pumps, this limitation is particularlysignificant in the context of an PDS unit fitted to a vehicle, in themanner previously described. That is because in many situations, the RDSmay “free-wheel” for cumulative periods of many hours. During this time,the unit may generate neither motor nor pump pressure, but wouldnevertheless continue to rotate at the same speed as the vehicle driveline to which it is integrally connected. This would predominantlyoccur, for example, during long runs on open highways with minimalchanges in traffic conditions or road topography. In such situations,the RDS may do no effective work for prolonged periods, and yetintroduce an inherent drag factor into the drive line, which ultimatelycompromises the efficiency of the power train of the vehicle.

[0010] Moreover, the basic problem is compounded in such conditionsbecause the drag factor itself is exacerbated when the pump/motor unitis operating in the neutral or free-wheeling mode. This is because theunit is not producing hydraulic pumping pressure sufficient to sustainthe film of hydrostatic pressurised oil required to minimise the effectof frictional drag between the components in direct rotational contactwith one another.

[0011] The foregoing discussion of the prior art is intended solely toplace the invention in an appropriate context, and allow a properappreciation of its technical significance. Any statements made in thisspecification about prior art information should not be construed asadmissions that such information is widely known, or forms part ofcommon general knowledge in the relevant field.

[0012] It is an object of the present invention to overcome orsubstantially ameliorate one or more of the deficiencies of the priorart, or at least to provide a useful alternative.

DISCLOSURE OF THE INVENTION

[0013] Accordingly, the invention provides a positive displacementhydraulic pump/motor assembly including:

[0014] a rotary cylinder block having a central axis and incorporating agenerally circular array of cylinders disposed in parallel relationshiparound the axis;

[0015] a corresponding plurality of axial pistons reciprocably disposedwithin the respective cylinders;

[0016] drive means to effect rotation of the cylinder block about thecentral axis;

[0017] a drive plate disposed at one end of the cylinder block to effectsequentially staggered reciprocation of the pistons in response torotation of the cylinder block;

[0018] a stationary valve plate disposed at an opposite end of thecylinder block, the valve plate having a valve face adapted for slidingrotational engagement with a complementary mating face formed on thecylinder block;

[0019] the valve plate further including at least one inlet port adaptedfor fluid communication with a source of hydraulic fluid and at leastone outlet port adapted for fluid communication with an hydraulic load;

[0020] the ports being disposed such that in use, hydraulic fluid isprogressively drawn into the cylinders in sequence as the respectivepistons are displaced away from the valve plate and subsequentlyexpelled from the cylinders as the pistons are progressively displacedtoward the valve plate;

[0021] the pump/motor assembly further including lift-off means beingselectively operable so as axially to displace the cylinder blockmarginally away from the valve plate, thereby to minimise rotationalresistance under predetermined operational conditions.

[0022] It will be appreciated that the same assembly may be used in onemode as a motor, and in another mode as a pump. It should therefore beunderstood that throughout the specification, these terns may be used inconjunction, or interchangeably. In each case, however, unless thecontext clearly dictates otherwise, any reference to configuration ofthe invention as a pump should be understood to include configurationsas a motor, and vice versa. It should also be understood that the inletand outlet ports may alternate in function according to the mode ofoperation of the unit.

[0023] It should further be understood that the terms “sealing”,“sealing engagement” and the like are intended to convey the sense ofprevention of excessive leakage past or through the relevant components.It will be appreciated, however, that in a system of this nature, aminimal level of leakage flow may persist, and indeed maybe desirablefor lubrication purposes, notwithstanding the fact that effectivesealing, in the intended sense, has been achieved.

[0024] Preferably, the drive means include a drive shaft, disposed incoaxial relationship with the cylinder block. Most preferably, the driveshaft extends through a complementary bore formed in the cylinder block.The assembly preferably further includes coupling means disposeddrivingly to connect the shaft to the cylinder block. The coupling meansmay be fixed or selectively releasable. In one embodiment, the shaft issplined or keyed to the cylinder block within the bore.

[0025] In a particularly preferred embodiment, the positive displacementpump/motor is a swash plate type unit. In this embodiment, the driveplate takes the form of a stationary swash plate, which is inclined withrespect to the central rotational axis of the cylinder block. Preferablyalso, the ends of the pistons remote from the valve plate include“followers” adapted to slide over the swash plate as the cylinder blockrotates. A hold-down plate is preferably disposed to capture thefloating ends of the pistons and retain the followers in sliding contactwith the swash plate. In alternative embodiment, however, springs orother suitable means may be used to retain the followers in contact withthe swash plate.

[0026] Preferably, the angle of inclination of the swash plate isselectively adjustable, to provide variable flow rate characteristics.In particular, the swash plate is preferably adapted to be selectivelyinclined in a positive or a negative sense, thereby enabling theassembly alternately to operate as a motor or a pump. Most preferably,the variable swash plate can also be oriented in an intermediate orneutral position, effectively normal to the central axis, such thatrotation of the cylinder block causes no movement of the pistons, henceinduces no net flow into or out of the cylinders through the ports, andtherefore causes no load on the system aside from a residual level ofinherent frictional drag.

[0027] In other embodiments, it will be appreciated that the inventionmay also be applied to a bent axis type hydraulic pump. In that case,connecting rods for the pistons are pivotably attached to a thrust plateadapted to rotate with the cylinder block. The invention may also beadaptable to other configurations of motors and pumps.

[0028] The lift-off means preferably include a selectively operablehydraulic lift-off cylinder effectively interposed between the cylinderblock and the valve plate, such that upon actuation, the cylinder blockis axially displaced marginally away from the valve plate. Preferably,the exposed end of the lift-off cylinder is in direct contact with athrust bearing, to facilitate continued rotation of the cylinder blockwith minimal frictional resistance. The lift-off cylinder would normallyonly be activated with the swash plate in the neutral position, whenthere is minimal internal pressure within the pump/motor assembly.Advantageously, this lift-off mechanism enables selective minimisationof frictional drag between the cylinder block and the valve plate insituations where sealing of the valve ports in order to prevent leakageflow is non-critical to performance, because the pump/motor unit isdoing no work.

[0029] Preferably, the pump/motor assembly further includes selectivelyvariable bias means disposed to apply a variable bias force urging therespective mating faces on the cylinder block and the valve plate intosealing engagement.

[0030] Preferably, the bias means include an hydraulic hold-down pistondisposed within a complementary hold down cylinder. An electronicallyactivated hydraulic valve actuator is preferably also provided toregulate pressure in the hold-down cylinder.

[0031] The hold-down cylinder is preferably supplied by pilot pressurefrom the pump/motor assembly. Alternatively, however, it may be suppliedfrom an external source of hydraulic fluid pressure. It will also beappreciated that the hold-down piston need not be hydraulic, but couldalternatively be operated by means of an electromagnetic solenoid, amechanical screw, or other suitable electrical, magnetic,electromagnetic, mechanical, hydraulic, pneumatic or hybridarrangements.

[0032] In the preferred embodiment, the hydraulic hold-down piston takesthe form of an annular sleeve disposed coaxially around the drive shaft,at the end of the cylinder block remote from the valve plate.

[0033] The valve actuator is preferably adapted to supply pressurisedoil into the hydraulic hold-down cylinder, and release oil from thatcylinder, according to the pressure and flow characteristics of thepump/motor assembly. In general, the system is ideally regulated suchthat under conditions of minimal motor or pump load, the pressure to thehold-down cylinder is released or substantially reduced so as tominimise frictional drag between the rotary cylinder block and thestationary valve plate. Conversely, under conditions of relatively highmotor or pump load, the supply pressure to the hold-down cylinder issubstantially increased, so as to minimise leakage from the ports at theinterface between the cylinder block and the valve plate.

[0034] This pressure regulation may be controlled in a linear, step-wiseor other manner in response to pressure changes at selected points inthe system. It may also be controlled wholly or in part according toother system parameters such as leakage flow, frictional drag,temperature changes or the like, as well as various combinations of suchparameters.

[0035] Preferably, in addition to the variable hold-down mechanism, theassembly includes a non-adjustable retention spring disposed to providea minimum level of substantially constant initial bias force sufficientto hold the cylinder block against the valve plate at a relatively lowthreshold pressure. In the preferred embodiment, the retention springtakes the form of a preloaded annular bevel or frusto-conical washerdisposed within an annular recess formed between the hold-down pistonand the cylinder block.

[0036] Preferably, the threshold force provided by the retention springis sufficient to allow the pump to supply a predetermined level of pilotpressure to the hydraulic hold-down cylinder with minimal leakage, at aninitial positive or negative (but non-zero) swash plate angle. Thisinitial swash plate angle for the desired level of pilot pressure ispreferably between 0.1 degrees and around 5.0 degrees, more preferablybetween 0.3 and around 1.0 degrees, and ideally around 0.5 degrees offthe neutral or zero position.

[0037] In a supplementary aspect, the invention preferably furtherincludes a releasable locking mechanism as part of the coupling means,to enable selective disengagement of the cylinder block and theassociated components in the rotational group, from the drive shaft.

[0038] More preferably, the locking mechanism includes a tapered lockingcollet disposed coaxially around the drive shaft. The locking collet isdesirably splined internally for engagement with a correspondinglysplined section of the shaft, so as to transmit rotary drive, whileaccommodating a limited degree of relative axial displacement betweenthe collet and the shaft.

[0039] Preferably, the hydraulic hold-down cylinder, in addition to itsfunction of applying a variable hold-down force, is configured topositively disengage the locking collet mechanism when the hydrauliclift-off cylinder is simultaneously de-pressurised.

[0040] The inner surface of the cylinder block is preferably tapered tomatch the outer surface profile of the locking collet. The outer surfaceof the locking collet is preferably also splined, for engagement withcomplementary splines formed on the inner surface of the cylinder blocksuch that upon axial engagement, the two components become mechanicallyinterlocked for conjoined rotation. In this way, the locking colletpositively transmits rotary drive from the shaft to the cylinder block,while still allowing a limited degree of axial displacement of therotational group along the shaft. This limited degree of axialdisplacement, with the splines engaged, preferably permits the cylinderblock to be alternately held positively against the valve plate by thehold-down cylinder and displaced marginally away from the valve plate bythe lift-off cylinder. Upon full axial disengagement, however, the shaftis able to spin independently of the surrounding rotational group.Advantageously, this mechanism selectively allows frictional andhydrodynamic drag to be further reduced in situations where thepump/motor assembly is not under load.

[0041] In a variation of this embodiment, other forms of interlockingengagement may be used as an alternative to splines, to selectivelytransmit drive between the shaft and the cylinder block. It will also beappreciated that the outer surface of the locking collet may not besplined or otherwise mechanically keyed for interlocking engagement withthe mating inner surface of the surrounding cylinder block. Rather, thelocking mechanism may rely upon progressive, albeit less positive,frictional engagement.

[0042] In one particularly preferred application, the invention isadapted for incorporation into an energy management system operable in adriving mode, a braking mode and a neutral mode, the energy managementsystem including:

[0043] energy accumulation means operable selectively to store andrelease energy through controlled receipt and release of hydraulicfluid;

[0044] a positive displacement hydraulic pump/motor assembly as definedabove, in fluid communication with the energy accumulation means;

[0045] an hydraulic reservoir in fluid communication with the pump/motorassembly;

[0046] and coupling means for coupling the pump/motor assembly to adrive shaft;

[0047] the system being arranged such that in the braking mode thepump/motor assembly retards the drive shaft by pumping hydraulic fluidinto the accumulation means, in the driving mode the pump/motor assemblysupplies supplementary power to the drive shaft using pressurisedhydraulic fluid from the accumulation means, and in the neutral mode thepump/motor assembly is effectively inoperative and exerts no substantialdriving or retarding influence on the drive shaft.

[0048] In one preferred implementation of this aspect of the invention,the drive shaft forms part of the drive train of a vehicle. Mostpreferably, the drive shaft extends through a complementary bore formedin the cylinder block, such that the drive shaft and the cylinder blockare coaxial, the pistons of the pump/motor assembly are uniformlydisposed in parallel relationship around the drive shaft. In thisembodiment, the coupling means preferably include a splined connectiondirectly between the vehicle drive shaft and the cylinder block. Inother embodiments, however, it will be appreciated that gears, clutches,or other coupling mechanisms may be interposed to transmit rotary drivebetween the vehicle drive train and the pump/motor unit. Suchtransmission may be mechanical, hydraulic, pneumatic or electromagnetic.It may also be permanently engaged or decouplable, manual or automatic,and may include constant or variable reduction ratios.

[0049] Preferably, the pump/motor assembly includes at least threeexternal ports to permit ingress and egress of hydraulic fluid, with afirst port communicating with an inlet of the hydraulic reservoir, asecond port communicating with an outlet of the hydraulic reservoir, anda third port communicating with the accumulation means. A heat exchangeris preferably disposed between the first port and the hydraulic fluidreservoir.

[0050] In one embodiment of the invention, a plurality of positivedisplacement axial piston pumps is arranged axially along the driveshaft. These pumps may be connected hydraulically to operate in series,parallel, or a combination of both.

[0051] Preferably, the energy management system includes a flow controlcircuit through which hydraulic fluid may be selectively directed, thecontrol circuit being adapted to provide a controllable resistanceenabling the pump/motor unit selectively to exert a retarding force onthe drive shaft when required, even if the accumulation means are fullycharged.

[0052] Preferably, the accumulation means include a gas/liquidaccumulator comprising a double-ended cylinder and a piston adapted tofloat sealingly within the cylinder. One side of the cylinder preferablycontains a compressible inert gas such as nitrogen, while the other sideof the cylinder is preferably connected hydraulically to the pump/motorunit. The accumulator is preferably thereby adapted to store energy bypumping hydraulic fluid into one side of the cylinder, so as to compressthe gas on the other side by displacement of the floating piston, andsubsequently to release that energy by expulsion of hydraulic fluid asthe compressed gas expands. In alternative embodiments, however, otherforms of accumulator, such as bladder or diaphragm type accumulators,may be readily substituted.

[0053] The assembly preferably includes a plurality of accumulators,which may be selectively connected in series, parallel or a combinationof both, as required.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] A preferred embodiment of the invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

[0055]FIG. 1 is a cross-sectional view showing a pump/motor assemblyincorporating a lift-off cylinder shown in the normal operating(deactivated) mode, according to a first embodiment of the invention;

[0056]FIG. 2 is a cross-sectional view of the pump/motor unit of FIG. 1,showing the lift-off cylinder in the unloaded (activated) mode;

[0057]FIG. 3 is a cross-sectional view of a pump/motor assembly, showinga hold-down cylinder activated in the normal operating or loadedconfiguration, according to a second aspect of the invention (NB—Forclarity, this version of the pump/motor unit as illustrated omits thelift-off cylinder shown in FIGS. 1 and 2);

[0058]FIG. 4 is a cross-sectional view of the pump/motor unit or FIG. 3,showing the hold-down cylinder in the unloaded or deactivatedconfiguration;

[0059]FIG. 5 is a cross-sectional view showing a pump/motor assemblyincorporating a hold-down cylinder, a lift-off cylinder and a taperedlocking collet mechanism in the normal operating (locked) mode accordingto a third embodiment of the invention;

[0060]FIG. 6 is a cross-sectional view of the pump/motor unit of FIG. 5,showing the locking collet mechanism in the unloaded (unlocked) mode;and

[0061]FIG. 7 is a diagrammatic perspective view showing a pump/motorassembly of the general type illustrated in FIGS. 1 to 6, incorporatedinto a road vehicle as part of a regenerative drive system (RDS),according to a further aspect of the invention.

PREFERRED EMBOIDMENT OF THE INVENTION

[0062] Referring initially to FIG. 1, the invention provides a positivedisplacement hydraulic pump/motor unit 1. The pump/motor unit includes astationary housing 2 and a cylinder block 3 supported within the housingfor rotation about a central axis 5. The block 3 incorporates a circulararray of hydraulic cylinders 6 uniformly disposed in parallelrelationship about the central rotational axis 5. A corresponding arrayof axial pistons 10 is reciprocably disposed within the respectivecylinders.

[0063] A central drive shaft 12 extends through a complementary bore 13formed in the cylinder block. The shaft is drivingly connected to thecylinder block 3 by coupling means including spline formations 14 toeffect rotation of the block about the central axis, as described inmore detail below.

[0064] A stationary drive plate in the form of swash plate 15 isdisposed at one end of the cylinder block (the right-hand end whenviewing the drawing). The swash plate is pivotably supported within thehousing, for adjustable movement within a predetermined range, about anaxis substantially normal to the rotational axis of the cylinder block.

[0065] A hold-down plate 16 is disposed to locate the free ends of thepistons remote from the valve plate in the appropriate relative spatialrelationship, while the end faces of the pistons are formed withfollowers 18 adapted to engage and slidably traverse the operativesurface of the swash plate. In this way, rotation of the cylinder blockeffects sequentially staggered reciprocation of the pistons, with theamplitude of piston travel being determined by the selected angle ofinclination of the swash plate.

[0066] A stationary valve plate 20 is disposed at the opposite end ofthe cylinder block (the left-hand end when viewing the drawings) and isrigidly connected to the housing. The valve plate includes a valve face21 adapted for sliding rotational engagement with a complementary matingvalve face 22 formed on the abutting end of the cylinder block. Thevalve plate includes inlet ports 24 adapted for communication with asource of hydraulic fluid, and outlet ports 26 adapted for fluidcommunication with an hydraulic load. A hold-down spring 42, disposedcoaxially around the drive shaft, resiliently urges the cylinder blockinto sealing engagement with the valve plate, with a predetermined forcebased on the calibrated preload of the spring.

[0067] The valving is arranged such that hydraulic fluid isprogressively drawn into the cylinders in sequence through the inletports as the pistons withdraw away from the valve plate and issubsequently expelled from the cylinders through the outlet ports as therespective pistons are progressively advanced toward the valve plate,under the influence of the swash plate.

[0068] The swash plate is pivotably supported within the housing suchthat the effective angle of inclination with respect to the rotationalaxis of the cylinder block is adjustable to provide selectively variableflow characteristics. In particular, the swash plate may be alternatelyinclined in a positive and a negative sense, thereby enabling theassembly selectively to operate as a motor or a pump. In this regard, itshould be appreciated that the particular valve ports which function asinlets to the cylinders of the pump/motor unit, and those which functionas outlets, will alternate according to the operational mode of theunit. Importantly, the swash plate can also be orientated in anintermediate or neutral position in a plane effectively normal to thecentral axis, such that rotation of the cylinder block produces noreciprocation of the pistons. In this mode, the pump/motor unit inducesno net fluid flow into or out of the cylinders, and consequentlytransfers no significant hydraulic load to the shaft.

[0069] The essential elements of construction and the basic principlesof operation are common to most positive displacement axial pistonhydraulic pumps, and being well understood by those skilled in the art,need not be described in more detail.

[0070] The pump/motor unit further includes a selectively operableactive release or lift-off mechanism in the form of an hydrauliclift-off piston 40, slidably disposed within an associated lift-offcylinder 41. The lift-off piston and cylinder are annular inconfiguration, and are positioned coaxially around the central driveshaft, effectively between the cylinder block and the valve plate.

[0071] In the normal operating mode, as shown in FIG. 1, the lift-offcylinder is depressurised through supply passage 44, and has no effecton the operation of the pump/motor unit. Upon actuation, however, thelift-off cylinder is pressurised again through supply passage 44,whereby the cylinder block is axially displaced away from the valveplate. This is the positively unloaded or actively released mode ofoperation, as represented in FIG. 2.

[0072] It will be appreciated that the pressure generated within thelift-off cylinder, in order to effect positive release of the cylinderblock, must be sufficient at least substantially to overcome thethreshold bias force of the retention spring 42 associated with thehold-down mechanism. It will also be appreciated that in the embodimentillustrated, the lift-off cylinder must activate against a thrustbearing (not shown) on the valve plate to allow for continued rotationof the cylinder block with the shaft with minimal resistance.

[0073] In practice, the extent of axial displacement produced by thelift-off cylinder may be marginal, and indeed may even be visuallyindiscernible. The significant result, however, is that the lift-offmechanism enables selective minimisation of frictional drag between thecylinder block and the valve plate in situations where the pump/motorunit is doing no work. Of course, the lift-off cylinder would normallyonly be activated with the swash plate in the neutral position, whenthere is consequently minimal internal pressure within the pump/motorassembly. Otherwise, the axial displacement of the cylinder block awayfrom the valve plate, albeit by only a small amount, may result inexcessive leakage of pressurised hydraulic fluid past the valve ports.

[0074] A second embodiment of the invention is shown in FIGS. 3 and 4,wherein like features are denoted by corresponding reference numerals.In this embodiment, the pump/motor unit incorporates a secondary aspectof the invention, in the form of controllable bias means disposed toapply a variable bias force urging the mating faces 21 and 22 on thevalve plate and the cylinder block respectively into sealing engagement.The bias means include an hydraulic hold-down piston 30 disposed withina complementary hold-down cylinder 32.

[0075] This active biasing mechanism is intended primarily to complementthe operation of the lift-off cylinder as illustrated in the firstembodiment of the invention shown in FIGS. 1 and 2. For clarity ofillustration, however, the lift-off mechanism is omitted from theembodiment shown in FIGS. 3 and 4.

[0076] Referring to FIGS. 3 and 4, the hold-down piston 30 takes theform of an annular sleeve disposed coaxially around the drive shaft, atthe front (right-hand when viewing the drawings) end of the cylinderblock, remote from the valve plate. The hold-down cylinder iscorrespondingly shaped, and is pressurised via a supply passage 35,which extends through the housing and the drive shaft as shown.

[0077] The pressure supplied to the hold-down cylinder is regulated byan electronic valve actuator 36 (represented diagrammatically in FIG. 3only) according to predetermined operational parameters related to thepressure and flow characteristics of the pump/motor assembly and othercharacteristics of the system, which will vary according to theapplication.

[0078] In general terms, the hold-down cylinder is regulated such thatunder conditions of relatively high motor or pump load, the supplypressure to the hold-down cylinder is maintained at a predeterminedlevel, sufficient to minimise leakage from the ports between the matingvalve faces at relatively high pump/motor pressures. This is the normaloperating mode for the unit, as shown in FIG. 3. Conversely, underconditions of minimal motor or pump load, the pressure to the hold-downcylinder is released or substantially reduced, so as to minimisefrictional drag at the sliding interface between the rotary cylinderblock and the stationary valve plate. This is the unloaded or passivelyreleased mode of operation, as illustrated in FIG. 4. In this mode,hydraulic fluid is exhausted from the hold-down cylinder via passage 35,as indicated by the directional arrows.

[0079] Within these limits, the pressure may be regulated by the valveactuator 36 in a linear, stepwise or other manner in response topressure changes at selected points in the system. It may also beregulated wholly or in part according to other parameters such asmeasured leakage flow, frictional drag, temperature changes or the like,as well as various combinations of such parameters on the basis of asuitable control system. This enables an optimum balance to be achieved,on a dynamic basis, between the need to minimise valve leakage on onehand, and the desirability of minimising internal friction andhydrodynamic drag on the other. In previously known pumps of this type,the cylinder block was simply biased toward the valve plate by a springapplying a substantially constant preload force (of the type shown inFIGS. 1 and 2), which if calibrated for maximum pressure conditionswould cause significant frictional drag and hence inefficiency at othertimes, or if calibrated for intermediate pressure levels, would allowexcessive leakage under high-pressure conditions.

[0080] In addition to the hydraulic hold-down piston and cylinderarrangement 30 and 32, the pump/motor unit ideally includes anon-adjustable retention spring mechanism, with a relatively low levelof substantially constant preload, calibrated to provide a minimalthreshold level of initial bias force. In the preferred embodiment asillustrated, the retention mechanism takes the form of a preloadedannular bevel or frusto-conical spring washer 39 disposed within acomplementary annular recess 40 formed between the hold-down piston andthe cylinder block. The bevel spring washer 39 is shown in a flattenedconfiguration in FIG. 1, corresponding to the normal operational orloaded mode, and in a marginally expanded configuration in FIG. 4,corresponding to the unloaded or passively released operational mode.

[0081] The initial bias force of the spring washer 39 is calibrated tohold the cylinder block against the valve plate with a relatively lowlevel of force in the unloaded or released mode of the hold-downcylinder, sufficient simply to allow the pump to begin supplying pilotpressure to the hold-down cylinder via passage 35 at an initial positiveor negative (but non-zero) swash plate angle. This initial swash plateangle corresponding to the desired level of pilot pressure is preferablybetween 0.1 and around 5.0 degrees, more preferably between 0.3 andaround 1.0 degrees, and ideally around 0.5 degrees off the neutral orzero position.

[0082] A further embodiment of the invention is illustrated in FIGS. 5and 6, where again similar features are denoted by correspondingreference numerals. In this version, it will be seen that cylinders 32and 41 from the respective embodiments previously described, areincorporated in combination into the pump/motor unit. However, thesecylinders operate somewhat differently than those previously described.Furthermore, the coupling means additionally include a releasablelocking mechanism 50, to enable selective disengagement of the cylinderblock and the associated components in the rotational group, from thecentral drive shaft.

[0083] The locking mechanism 50 includes a tapered locking collet 52,which is splined internally for engagement with the corresponding splineformations 14 on the drive shaft. The splined connection transmitsrotary drive while accommodating a limited degree of relative axialdisplacement between the locking collet 52 and the shaft. The innersurface 54 of the cylinder block is shaped to match the tapered externalsurface profile of the locking collet and these tapered surfaces alsoincorporate complementary axially interengageable spline formations 55.Additionally, and annular sleeve 60 incorporating a shoulder flange 62is interposed between the hold-down piston 30 and the locking collet 52

[0084] In this embodiment, in the normal operating (locked) mode asshown in FIG. 5, the cylinder 41 is pressurised via passage 44, whichurges that piston to the right (when viewing the drawings) whilesimultaneously urging the cylinder block to the left, toward the valveplate. As the piston 40 advances it drives the locking collet 52, theannular sleeve 60 and the piston 30 axially and in unison to the right,away from the valve plate. In order to accommodate this movement,cylinder 32 is simultaneously depressurised via passage 35. However,when the annular end face 64 of the piston 30 abuts the adjacent end ofthe cylinder 32, no further axial displacement is possible. From thispoint, further pressurisation of cylinder 41 through passage 44progressively drives the cylinder block into sealing engagement with thevalve plate in order to perform the hold-down function. At the sametime, it will be appreciated that the piston 40 and cylinder 41 operateto retain the splines 55 on the locking collet and the surroundingcylinder block in interlocking engagement. In conjunction with thesplined connection between the locking collet and the shaft, thiseffectively locks the cylinder block, the locking collet and the shafttogether for conjoined rotation.

[0085] In order to unload the cylinder block from the valve plate and atthe same time to positively unlock the cylinder block from the shaft,the cylinder 41 is vented via passage 44 while the cylinder 32 issimultaneously pressurised via passage 35, as shown in FIG. 6. In thisconfiguration, the end face and 66 of the annular sleeve 60 engages theadjacent face of the locking collet, and drives the locking colletaxially toward the valve plate 20 (to the left when reviewing thedrawings).

[0086] During an initial phase of displacement, the locking colletcarries the cylinder block with it. However, once the cylinder blockfirmly engages the valve plate, no further axial displacement ispossible. Upon further pressurisation of cylinder 32, the piston 30,sleeve 60, and collet 52 are displaced progressively toward the valveplate while the cylinder block is held back, until the piston 40 reachesthe end of its travel, at which point no further axial displacement ofthe locking collet is possible. At this point, the previouslyintermeshing tapered splines 55 on the locking collet and thesurrounding cylinder block are disengaged, to permit relative rotationbetween the rotational group and the drive shaft. In this position, thecylinder block is both unloaded and unlocked, as represented by theconfiguration shown in FIG. 6. In this version, it will also be notedthat coil springs 68 are incorporated within the cylinders to bias therespective pistons into contact with the swash plate, thereby minimisingfree play and noise within the pump/motor unit, particularly whenoperating in the unloaded or unlocked mode.

[0087] It will be appreciated that this mechanism, when unlocked insituations where the pump/motor assembly is not under load, effectivelyeliminates frictional drag between the cylinder block and the valveplate. It also eliminates drag between the cylinder block and otherstationary elements within the housing. This feature is particularlyadvantageous in applications where the cylinder block rotates within anoil bath and consequently, the hydrodynamic dragon the rotational groupis relatively high. In some larger scale applications, however,particularly where there is mechanical interlocking engagement betweenthe drive shaft and the rotational group, it may be desirable to providean additional control mechanism adapted to spin the cylinder block up tothe approximate speed of the drive shaft, so as to minimise transientshock loads at the point of initial engagement of the locking mechanism.

[0088] The present invention in its various forms as illustrated isparticularly well adapted for implementation as part of a regenerativedrive system (RDS) of the type previously described by the presentapplicant in the earlier application No. PCT/AU99/00740. An example ofone such implementation in a vehicle chassis 70 is illustrated in FIG.7.

[0089] Referring to FIG. 7, the hydraulic pump/motor assembly 1, asdescribed above, forms part of an energy management system 72, whereinthe drive shaft 12 of the pump/motor unit is connected to the drive lineor power train 74 of the vehicle via yokes 76 and 78. In situ, theseyokes form parts of universal joints at the respective ends of the driveshaft. In this way, the drive shaft of the pump/motor unit becomesserially connected with, and an integral part of, the drive line of thevehicle, thereby obviating the need for gearboxes, chain drives, beltdrives, or other intermediate transmission mechanisms. This makes theunit efficient, compact, reliable, and readily retrofitable to existingvehicles.

[0090] In this implementation, the system further includes energyaccumulation means in the form of a pair of gas/liquid accumulators 80,each comprising a double-ended cylinder 82 and a piston (not shown)adapted sealingly to float within the cylinder. One side of eachcylinder contains a compressible inert gas such as nitrogen, while theother side of the cylinder is in fluid communication with the pump/motorunit via hydraulic lines. Each accumulator is thereby adapted to storeenergy by receiving pressurised hydraulic fluid into one side of thecylinder so as to compress the gas on the other side, and adaptedsubsequently to release that energy by expulsion of the hydraulic fluidas the compressed gas is allowed to expand. This method of energyaccumulation and regeneration is well understood by those skilled in theart, and is described in more detail in PCT/AU99/00740. Again, however,it should be emphasised that alternative forms of energy accumulatorsuch as bladder or diaphragm type accumulators can readily besubstituted and further, that any suitable number of accumulators may beused.

[0091] In use, the system is selectively operable in any one of threeprimary modes. In a first braking mode, the system operates to retardthe drive shaft of the vehicle by pumping hydraulic fluid into theaccumulators and thereby compressing the contained gas medium.Alternately, the system is operable in a driving mode to supplysupplementary power to the drive shaft of the vehicle using thepressurised hydraulic fluid from the accumulators. In the braking mode,it will be appreciated that the hydraulic pump/motor unit operates as apump powered by the vehicle drive shaft, whereas in the driving mode,the unit operates as a motor powered by pressurised hydraulic fluid fromthe accumulators. The system is also operable in a third neutral or“free wheeling” mode, whereby the drive shaft of the vehicle issubstantially unaffected by the pump/motor unit, aside from any residualfrictional drag which the present invention aims to minimise.

[0092] In the neutral or free wheeling mode, the cylinder block may bemarginally displaced from the valve plate by selective actuation of thelift-off cylinder, to minimise a key source of frictional power losswithin the pump/motor unit. Simultaneously or alternatively, the lockingcollet may be selectively disengaged. As previously described, thisavoids the need for the cylinder block to rotate in unison with thedrive shaft when not required. The effect of this disengagement is tosubstantially eliminate a primary source of hydrodynamic drag, whicharises by virtue of the fact that the rotational group normally spins inan oil bath contained within the housing.

[0093] The three primary operational modes are controlled by the angleof inclination of the swash plate 15, in the manner previouslydescribed. This angle, together with the related operation of thehold-down cylinder, lift-off cylinder and locking collet mechanism, isregulated by a computer controlled electronic RDS management system 90.The RDS management system is programmably responsive to a predeterminedseries of system parameters such as accelerator, brake and clutch pedalpositions, engine speed, gear selection, engine manifold pressure, swashplate position, drive line torque, accumulator pressure and hydraulicpump/motor pressure. The system may also be pre-programmed withtopographical mapping and terrain data, thereby enabling it effectivelyto anticipate inclines and declines as well as stopping and accelerationpoints on known routes, and optimise the performance of the RDS on thatbasis. Again, the general operating principles in this respect aredescribed in more detail in PCT/AU99/00740.

[0094] The RDS unit as illustrated is positioned in the central driveline of the vehicle, immediately downstream of the engine 92 and gearbox94. This is advantageous, because in that position, the system can bereadily retrofitted to existing vehicles by replacement of a standardsection of the original drive line. It should be understood, however,that the unit may alternatively be positioned between the engine andgearbox, for example in direct connection with the crank shaft. Asanother alternative, it may be incorporated into a gearbox or enginecasing. It may also be positioned downstream of one or both of thedifferentials 98, and may even be incorporated into an axle. In thatcase, it will be apparent that several RDS units may be incorporatedinto multiple drive line sections, axles or stub axles.

[0095] While the particular implementation of the invention shown in thedrawings is described predominantly in the context of heavy road goingvehicles, it will also be appreciated that regenerative drive systems ofthis type can be readily adapted to practically any environment in whichit is advantageous to store excess mechanical energy at some time, foruse at a later time. Typical examples of other potential applicationsinclude elevators and lifts, escalators and travelators, conveyors,cranes and other lifting devices, as well as other forms oftransportation such as rail, shipping and aeronautical applications.

[0096] The present invention provides an efficient and effectivemechanism for reducing frictional or hydrodynamic drag under off-loadconditions in axial piston hydraulic pumps and motors, of almost anyconfiguration and in virtually any application. Moreover, this can beachieved without compromising sealing performance under heavy load orhigh pressure conditions. The benefits of this are particularlysubstantial in the context of regenerative drive systems, wherein thepump/motor unit may be required to operate in a neutral or freewheelingmode for prolonged periods, and any residual frictional drag during suchperiods can impact materially on the overall efficiency of the system.In these respects, the invention represents both a practical and acommercially significant improvement over the prior art.

[0097] Although the invention has been described with reference tospecific examples, it will be appreciated by those skilled in the artthat the invention may be embodied in many other forms.

1. A positive displacement hydraulic pump/motor assembly including: arotary cylinder block having a central axis and incorporating agenerally circular array of cylinders disposed in parallel relationshiparound the axis; a corresponding plurality of axial pistons reciprocablydisposed within the respective cylinders; drive means to effect rotationof the cylinder block about the central axis; a drive plate disposed atone end of the cylinder block to effect sequentially staggeredreciprocation of the pistons in response to rotation of the cylinderblock; a stationary valve plate disposed at an opposite end of thecylinder block, the valve plate having a valve face adapted for slidingrotational engagement with a complementary mating face formed on thecylinder block; the valve plate further including at least one inletport adapted for fluid communication with a source of hydraulic fluidand at least one outlet port adapted for fluid communication with anhydraulic load; the ports being disposed such that in use, hydraulicfluid is progressively drawn into the cylinders in sequence as therespective pistons are displaced away from the valve plate andsubsequently expelled from the cylinders as the pistons areprogressively displaced toward the valve plate; the pump/motor assemblyfurther including lift-off means being selectively operable so asaxially to displace the cylinder block marginally away from the valveplate, thereby to minimise rotational resistance under predeterminedoperational conditions.
 2. A pump/motor assembly according to claim 1,wherein the drive means include a drive shaft, disposed in coaxialrelationship with the cylinder block.
 3. A pump/motor assembly accordingto claim 1, wherein the drive shaft extends through a complementary boreformed in the cylinder block.
 4. A pump/motor assembly according toclaim 3, further including coupling means disposed drivingly to connectthe shaft to the cylinder block.
 5. A pump/motor assembly according toclaim 4, wherein the coupling means include a splined or keyedconnection between the shaft and the bore of the cylinder block.
 6. Apump/motor assembly according to claim 4, wherein the coupling means arefixed.
 7. A pump/motor assembly according to claim 4, wherein thecoupling means are selectively releasable.
 8. A pump/motor assemblyaccording to claim 1, wherein the drive plate takes the form of astationary swash plate, which is inclined or inclinable with respect tothe central rotational axis of the cylinder block.
 9. A pump/motorassembly according to claim 8, wherein the ends of the pistons remotefrom the valve plate include followers adapted to slide over the swashplate as the cylinder block rotates.
 10. A pump/motor assembly accordingto claim 9, wherein a hold-down plate is disposed to locate the floatingends of the pistons and retain the followers in sliding contact with theswash plate.
 11. A pump/motor assembly according to claim 9, furtherincluding springs to facilitate retention of the followers in contactwith the swash plate.
 12. A pump/motor assembly according to claim 8,wherein the angle of inclination of the swash plate is selectivelyadjustable, to provide selectively variable flow rate characteristics.13. A pump/motor assembly according to claim 12, wherein the swash plateis adapted to be selectively inclined in a positive or a negative sense,thereby enabling the assembly alternately to operate as a motor or apump.
 14. A pump/motor assembly according to claim 13, wherein the swashplate can also be oriented in an intermediate or neutral positioneffectively normal to the central axis, such that rotation of thecylinder block causes no axial movement of the pistons, hence induces nonet flow into or out of the cylinders through the ports, and thereforeinduces no substantial load or drive on the shaft.
 15. A pump/motorassembly according to claim 1, wherein the pump/motor unit is a bentaxis type hydraulic pump, further including piston connecting rodspivotably attached to a thrust plate adapted to rotate with the cylinderblock.
 16. A pump/motor assembly according to claim 1, wherein thelift-off means include a selectively operable hydraulic lift-offcylinder effectively interposed between the cylinder block and the valveplate, such that upon actuation, the cylinder block is axially displacedmarginally away from the valve plate.
 17. A pump/motor assemblyaccording to claim 16, wherein one end of the lift-off cylinder is indirect contact with a thrust bearing, to facilitate continued rotationof the cylinder block with minimal frictional resistance with thelift-off cylinder activated.
 18. A pump/motor assembly according toclaim 16, wherein the lift-off cylinder is adapted to be activated withthe swash plate in the neutral position, when there is minimal hydraulicpressure within the pump/motor assembly, thereby enabling selectiveminimisation of frictional drag between the cylinder block and the valveplate in situations where sealing of the valve ports in order to preventleakage flow is non-critical to performance, because the pump/motor unitis doing no work.
 19. A pump/motor assembly according to claim 1,further including selectively variable bias means disposed to apply avariable bias force urging respective mating faces on the cylinder blockand the valve plate into sealing engagement.
 20. A pump/motor assemblyaccording to claim 19, wherein said bias means include an hydraulichold-down piston disposed within a complementary hold-down cylinder. 21.A pump/motor assembly according to claim 20, further including a controlactuator disposed to regulate pressure in the hold-down cylinder.
 22. Apump/motor assembly according to claim 20, wherein the hold-downcylinder is supplied by pilot pressure from the pump/motor unit.
 23. Apump/motor assembly according to claim 20, wherein the hold-downcylinder is supplied from a supplementary external source of hydraulicfluid pressure.
 24. A pump/motor assembly according to claim 20, whereinthe hold-down piston takes the form of an annular sleeve disposedcoaxially around the drive shaft, at an end of the cylinder block remotefrom the valve plate.
 25. A pump/motor assembly according to claim 21,wherein the control actuator is adapted to regulate the hold-downcylinder according to predetermined pressure and flow characteristics ofthe pump/motor assembly.
 26. A pump/motor assembly according to claim25, wherein the hold-down cylinder is regulated such that underconditions of minimal motor or pump load, the pressure to the hold-downcylinder is released or substantially reduced so as to minimisefrictional drag between the rotary cylinder block and the stationaryvalve plate, and such that under conditions of relatively high motor orpump load, the pressure to the hold-down cylinder is substantiallyincreased, so as to minimise leakage between the cylinder block and thevalve plate.
 27. A pump/motor assembly according to claim 26, whereinthe hold-down cylinder is controlled in a linear, step-wise or othermanner in response to pressure changes at selected points in the system.28. A pump/motor assembly according to claim 27, wherein the hold-downcylinder is further controlled wholly or in part according to othersystem parameters including one or more of: leakage flow; frictionaldrag; temperature changes; or combinations of such parameters.
 29. Apump/motor assembly according to claim 20, wherein the bias means takesa form selected from the group comprising: an electromagnetic solenoid;a mechanical screw; an electrical actuator; a magnetic actuator; amechanical linkage; or an hydraulic, pneumatic, electrical or mechanicalhybrid actuating arrangement.
 30. A pump/motor assembly according toclaim 20, further including a retention spring disposed to provide aminimum threshold level of substantially constant initial bias forcesufficient to hold the cylinder block against the valve plate atrelatively low pump/motor pressure levels, independently of theselectively variable bias means.
 31. A pump/motor assembly according toclaim 30, wherein the retention spring takes the form of a preloadedannular bevel or frusto-conical washer disposed within an annular recessformed between the hold-down piston and the cylinder block.
 32. Apump/motor assembly according to claim 31, wherein the threshold forceprovided by the retention spring is sufficient to allow the pump tosupply a predetermined level of pilot pressure to the hydraulichold-down cylinder with minimal leakage, at an initial non-zero swashplate angle.
 33. A pump/motor assembly according to claim 32, whereinthe initial swash plate angle for the predetermined level of pilotpressure is between 0.1 degrees and around 5.0 degrees.
 34. A pump/motorassembly according to claim 33, wherein the initial swash plate anglefor the predetermined level of pilot pressure is between 0.3 degrees andaround 1.0 degree.
 35. A pump/motor assembly according to claim 34,wherein the initial swash plate angle for the predetermined level ofpilot pressure is around 0.5 degrees off the neutral or zero position.36. A pump/motor assembly according to claim 1, wherein the couplingmeans include a releasable locking mechanism, adapted to enableselective disengagement of the cylinder block from the drive shaft. 37.A pump/motor assembly according to claim 36, wherein the releasablelocking mechanism includes a tapered locking collet disposed coaxiallyaround the drive shaft.
 38. A pump/motor assembly according to claim 37,wherein the locking collet is splined internally for engagement with acorrespondingly splined section of the shaft, so as to transmit rotarydrive, while accommodating a limited degree of relative axialdisplacement between the collet and the shaft.
 39. A pump/motor assemblyaccording to claim 38, when dependent upon any one of claims 20 to 37,wherein the hydraulic hold-down cylinder, in addition to its function ofapplying a variable hold-down force, is configured positively todisengage the locking collet mechanism when the hydraulic lift-offcylinder is simultaneously de-pressurised.
 40. A pump/motor assemblyaccording to claim 39, wherein an inner surface of the cylinder block istapered to match a corresponding outer surface profile of the lockingcollet.
 41. A pump/motor assembly according to claim 40, wherein theouter surface of the locking collet is also splined, for engagement withcomplementary splines formed on the inner surface of the cylinder blocksuch that upon axial engagement, the two components become mechanicallyinterlocked for conjoined rotation.
 42. A pump/motor assembly accordingto claim 41, wherein the locking collet positively transmits rotarydrive from the shaft to the cylinder block while allowing a limiteddegree of axial displacement of the cylinder block along the shaft, saidlimited degree of axial displacement, with the splines engaged,permitting the cylinder block to be alternately held positively againstthe valve plate by the hold-down cylinder and displaced marginally awayfrom the valve plate by the lift-off cylinder, whereas upon full axialdisengagement, the shaft is able to spin independently of the cylinderblock.
 43. An energy management system operable in a driving mode, abraking mode and a neutral mode, said energy management systemincluding: energy accumulation means operable selectively to store andrelease energy through controlled receipt and release of hydraulicfluid; a positive displacement hydraulic pump/motor assembly as definedin any one of the preceding claims, in fluid communication with theenergy accumulation means; an hydraulic reservoir in fluid communicationwith the pump/motor assembly; and coupling means for coupling thepump/motor assembly to a drive shaft; the system being arranged suchthat in the braking mode the pump/motor assembly retards the drive shaftby pumping hydraulic fluid into the accumulation means, in the drivingmode the pump/motor assembly supplies supplementary power to the driveshaft using pressurised hydraulic fluid from the accumulation means, andin the neutral mode the pump/motor assembly is effectively inoperativeand exerts no substantial driving or retarding influence on the driveshaft.
 44. An energy management system according to claim 43, whereinthe drive shaft forms part of the drive train of a vehicle.
 45. Anenergy management system according to claim 43, wherein the drive shaftextends through a complementary bore formed in the cylinder block, suchthat the drive shaft and the cylinder block are coaxial, with thepistons of the pump/motor assembly being uniformly disposed in parallelrelationship around the drive shaft.
 46. An energy management systemaccording to any one of claims 43 to 45, wherein the coupling meansinclude a connection adapted to transmit drive directly between thevehicle drive shaft and the cylinder block.
 47. An energy managementsystem according to claim 46, wherein said connection includes splineformations formed respectively in the drive shaft and the cylinderblock.
 48. An energy management system according to claim 43, whereinsaid connection includes a transmission interposed to transmit rotarydrive between the vehicle drive train and the pump/motor unit.
 49. Anenergy management system according to claim 48, wherein saidtransmission includes mechanical, hydraulic, pneumatic orelectromagnetic transmission elements.
 50. An energy management systemaccording to claim 48, wherein said transmission is adapted to thepermanently engaged.
 51. An energy management system according to claim48, wherein said transmission is selectively decouplable.
 52. An energymanagement system according to claim 43, wherein the pump/motor assemblyincludes at least three external ports to permit ingress and egress ofhydraulic fluid, with a first port communicating with an inlet of thehydraulic reservoir, a second port communicating with an outlet of thehydraulic reservoir, and a third port communicating with theaccumulation means.
 53. An energy management system according to claim43, wherein a heat exchanger is disposed between the first port and thehydraulic fluid reservoir.
 54. An energy management system according toclaim 43, wherein a plurality of said pump/motor assemblies is arrangedaxially along the drive shaft, and connected hydraulically to operate inseries, parallel, or a combination of both.
 55. An energy managementsystem according to claim 43, further including a flow control circuitthrough which hydraulic fluid may be selectively directed, the controlcircuit being adapted to provide a controllable resistance enabling thepump/motor unit selectively to exert a retarding force on the driveshaft when required, irrespective of the charge state of theaccumulation means.
 56. An energy management system according to claim43, wherein the accumulation means include a gas/liquid accumulatorcomprising a double-ended cylinder and a piston adapted to floatsealingly within the cylinder.
 57. An energy management system accordingto claim 56, wherein one side of the cylinder contains a compressibleinert gas, while the other side of the cylinder is connectedhydraulically to the pump/motor assembly, whereby the accumulator isadapted to store energy by pumping hydraulic fluid into one side of thecylinder, so as to compress the gas on the other side by displacement ofthe floating piston, and subsequently to release that energy byexpulsion of hydraulic fluid as the compressed gas expands.
 58. Anenergy management system according to claim 43, wherein the accumulationmeans include a bladder or diaphragm type accumulator.
 59. An energymanagement system according to claim 43, wherein the accumulation meansinclude a plurality of accumulators connected in series.
 60. An energymanagement system according to claim 43, wherein the accumulation meansinclude a plurality of accumulators connected in parallel.